1. Field of the Invention
The present invention relates generally to an apparatus and method for measuring a borehole and, more particularly, to an apparatus and method for acoustically measuring the caliper of a borehole, and the standoff of a drilling assembly from a borehole wall during a drilling operation.
2. Description of Related Art
Modern petroleum drilling and production operations require a great deal of information relating to underground conditions. Such information typically includes characteristics of the earth formations traversed by a well, in addition to data on the size and configuration of the borehole itself. Among the characteristics of the earth formation of interest to drillers are the porosity and density of the rock or strata surrounding the borehole. However, the processes often employed to measure these characteristics are subject to important errors unless information on the borehole size and configuration is also taken into account in their determination.
The collection of information on downhole conditions, often referred to as "logging" in the art, may be accomplished in several ways. A sonde, or probing tool, often having a number of detecting and sensing devices for measuring various downhole parameters, may be lowered into the borehole on the end of a cable, or wireline. The cable serves to control the position of the sonde and conveys information collected by the detectors and sensors to the surface where the data can be evaluated. Such wireline techniques may be used for measuring the caliper of a borehole by incorporating mechanical calipers in a sonde. The calipers extend out from the sonde and contact the wall of the borehole to measure its dimensions. It is also known to incorporate acoustic devices in a wireline sonde for measuring the caliper of the borehole.
While wireline logging is useful in collecting information on underground formations, it has a number of drawbacks. In order to insert the sonde in the borehole, the drillstring must be removed, resulting in considerable cost and downtime for the driller. In addition, such techniques do not afford data collection during the drilling operation. Moreover, mechanical calipers may scratch or damage the borehole or well casing, and are not suitable for obtaining measurements during the drilling process.
In recent years increasing emphasis has been placed on the collection of data on downhole conditions during the drilling process. In addition to information on weight on bit, torque on bit, and direction of drilling, such information typically includes the size and configuration of the borehole, the position of the drilling assembly in the borehole, and the movement of the drill bit and the drillpipe in the hole. By collecting and processing such information during the drilling process, the driller can modify or correct key parameters of the operation to optimize performance. Schemes for collecting measurements of downhole conditions and movement of the drilling assembly during the drilling operation have come to be known collectively as "measurement while drilling" techniques, or "MWD.[ Similar techniques, concentrating more on measurement of formation parameters than on movement of the drilling assembly, have recently been differentiated by the name "logging while drilling" or "LWD." While distinctions between these types of data collection may exist, the terms MWD and LWD are often used interchangeably, and the term LWD, as used throughout the present description, should be understood to include both the collection of formation and borehole information, as well as data on movement of the drilling assembly.
In known MWD and LWD techniques, detecting and sensing devices are positioned on a downhole tool, or "bottom hole assembly," abbreviated "BHA," above the drill bit. The configuration of the tool may vary depending upon the data being collected, but such tools typically include devices for determining the neutron porosity of earth formations, employing a nuclear source housed in the tool. To determine the density of such formations, the tool often includes devices which emit energy, such as gamma rays, to the formation surrounding the tool, and receive and detect some part of the energy reflected by the formation. Acoustic devices have recently been included in the tools to collect information on the size, or caliper of the hole as well as on standoff.
Accurate caliper and standoff measurements collected in LWD operations are important for a number of reasons. Because the density and porosity measurements are used to calculate characteristics of the earth formation based on assumptions about the size and configuration of the borehole, the caliper and standoff measurements collected by acoustic devices are useful in correcting the density and porosity measurements where the actual borehole conditions differ from those assumed in the calculations. Thus, accurate caliper and standoff measurements are key to obtaining and interpreting reliable density and porosity data. In addition, knowing the caliper of the borehole allows the driller to estimate the amount of cement required to fill the annular volume between the casing and the formation upon completion of the well. Also, such information can permit the driller to interpret how the drill bit or drillpipe is behaving during drilling. Because unusual movements of the drillpipe, such as whirling, sticking, and lateral bouncing, can have a very detrimental effect on the drill bit and drillpipe, this information can be extremely useful to the driller in mitigating tool wear and associated operational costs.
Due to the unsuitability of mechanical calipers to LWD applications, a number of indirect methods of determining borehole diameter have been proposed. These methods generally infer borehole caliper from other measurements taken in the well. Examples of such techniques are described in the following technical papers: D. Best, P. Wraight, and J. Holenka, "An Innovative Approach to Correct Density Measurements While Drilling for Hole Size Effect," Society of Professional Well Log Analysts Thirty-First Annual Logging Symposium Transactions, Jun. 24-27, 1990, paper G; W. C. Paske, M. V. Rao, J. R. Twist, S. G. Mack, and R. L. Spross, "Theory and Implementation of a Borehole Caliper Measurement Made While Drilling," Society of Petroleum Engineers Annual Technical Conference, Sep. 23-26, 1990, paper SPE 20562; and R. A. Rosthal, D. L. Best, and B. Clark, "Borehole Caliper While Drilling from a 2-MHZ Propagation Tool," Society of Petroleum Engineers Annual Technical Conference, Oct. 6-9, 1991, paper SPE 22707. However, because such techniques employ indirect methods rather than direct measurement of borehole caliper, the resulting data are of limited accuracy.
Known LWD tools employing acoustic devices for collecting caliper and standoff information generally either include a single acoustic device, or two such devices positioned in diametrically or azimuthally opposed locations on the tool. In operation, the acoustic device emits an ultrasonic signal in response to a timed voltage pulse. The signal travels through the drilling fluid, or mud, surrounding the tool, is reflected from the wall of the earth formation comprising the borehole, and returns to the tool. A detector on the tool receives the reflected signal, and electronic circuitry records the time of reception. By measuring or estimating the speed at which sound propagates through the drilling fluid, referred to as the acoustic velocity of the fluid, the time required for the signal to travel to the borehole wall and back to the tool can be used to calculate the standoff of the tool from the wall. Similar calculations, performed very rapidly as the drillstring turns in the borehole, provide information on the caliper and configuration of the hole. A caliper of this type is described in a paper by J. J. Orban, M. S. Dennison, B. M. Jorion, and J. C. Mayes, entitled "New Ultrasonic Caliper for MWD Operations," SPE/ADC Drilling Conference, Mar. 11-14 1991, paper SPE/ADC 21947.
However, such tools do not provide accurate caliper measurements where the borehole is elliptical, or when the tool is not centered in the borehole. Such eccentricity within the borehole is very common due to movement of the drill bit and drillpipe, particularly in directional or horizontal drilling operations. Moreover, in many drilling operations, rotation of the drill bit is not carried out by rotating the drillstring from the surface, but by the use of a downhole motor, or "mud motor," which forces the drill bit to rotate by the pressurized flow of drilling fluid through the drillpipe. In such cases, the drillstring may rarely be concentric with the borehole, and single or double transducer caliper systems do not provide sufficiently accurate measurements. Finally, while measurements from single or double transducer caliper systems may be averaged to reduce error, this is only possible for measurements taken during rotation of the drillstring. Errors in measurements taken during tripping into and out of a well, during which time the drillstring is not rotating, cannot be reduced by such averaging.
The present invention is directed to overcoming or minimizing the drawbacks of the existing techniques set forth above. In particular, the invention is directed to a tool and method for accurately and reliably measuring the dimensions of a borehole during a LWD operation.